First you have to open the reel. They come apart pretty easily.
Then hunt in the PVC, ABS, CPVC piping at (somewhere - Home Depot, Lowes, Ace Hardware) for a piece of pipe that has an I.D. big enough to accept the smaller hub from the reel. But hopefully not too big.
I think CPVC will be the thinnest. So that's my first choice.
I expect you'll have to reduce the smaller hub and ream out the larger one. But one should be able to make a fixed reel of any arbitrary width with the pipe extending the middle. And as long as the ends are cut square then one can expect to butt the bottom flush and the unit will be square as well.
Obviously the width should be sized to accept the 5" dimensions of the film. I don't know if the 5" side of 4x5 and the 5" side of 5x7 are exactly the same or not.
This should be a reel that you could push sheets of film into the spiral slots if you tried. I suspect the plastic pipe glues will work to hold it all together.
Now, you cannot just push sheets of film into the reel because they'll move and overlap, ruining your film during processing. Also, the 5" edge presents a giant lip for the liquid to pull the film out of the slots. So the next piece is crucial.
We'll need stabilizers between sheets of film, and at the front edge of the first sheet as well as the trailing edge of the last sheet.
Phototherm made the 120 stabilizers out of 1" mini blinds. So this is a pretty good material. Steal the bottom one or two blades, down where a bunch pack together, from a regular 1" plastic mini blind.
The stabilizers need to be cut to length so they'll slip into the spiral like a crossbar.
Obviously "just a piece of mini blind" will not prevent the sheet from slipping, so we'll need to use them as pockets somehow. My thinking is to take two pieces cut to length, then sew them together so that they can clamp the film edge.
Sewing right down the middle will make the pocket too big, so my suspicion is that it will take two seams to make adequate pockets.
One stabilizer needs to go into the spool first to hold the leading edge of the first sheet. Then the first sheet needs to be loaded. After the first sheet another stabilizer with the trailing edge of the first sheet secured. A second sheet goes in with its leading edge inserted into the back of the stabilizer bar. Then a stabilizer bar bringing up the rear.
I'm certain that two sheets will fit, and three might fit. Three sheets is only 21 inches into the spiral, but sheet film is thick compared to roll film, so getting it to insert all the way to the center is foolhardy.
Does any of this make sense? It's tough to describe a vision in words.